1. Field of the Invention
This invention relates generally to the field of compression release retarders for internal combustion engines equipped with turbochargers. More particularly it relates to a method and apparatus to control the intake manifold air pressure during compression release engine retarding thereby controlling the cylinder pressure and the retarding horsepower developed by the engine retarder. Such control of the intake manifold air pressure limits the loads induced in the engine valve train and in the piston, connecting rod and crankshaft mechanism during compression release engine retarding.
2. Prior Art
Engine retarders of the compression release type are well-known in the art and are commonly used to augment the service brakes on commercial vehicles. Such engine retarders are designed to convert, temporarily, an internal combustion engine of the spark ignition or compression ignition type into an air compressor so as to develop a retarding horsepower which may be a substantial portion of the operating horsepower developed by the engine.
The compression release engine retarder of the type disclosed in Cummins U.S. Pat. No. 3,220,392 employs an hydraulic system wherein the motion of a master piston controls the motion of a slave piston which, in turn, opens the exhaust valve of the internal combustion engine near the end of the compression stroke whereby the work done in compressing the intake air is not recovered during the expansion or "power" stroke, but, instead, is dissipated through the exhaust and cooling systems of the vehicle. The master piston is customarily driven by a pushtube controlled by a cam on the engine camshaft which may be associated with the fuel injector of the cylinder involved or with the intake or exhaust valve of another cylinder.
One of the advantages of the compression release retarder of the type disclosed in the Cummins U.S. Pat. No. 3,220,392 is that it may be incorporated into an existing engine without redesign or reconstruction of the engine. This advantage distinguishes the Cummins type retarder from other compression release retarders which require extra cams or cam profiles (see Pelizzoni U.S. Pat. No. 3,786,792; Dreisin U.S. Pat. No. 3,859,970; Jonsson U.S. Pat. No. 3,367,312; and Cartledge U.S. Pat. No. 3,809,033). However, the use of existing valve train components to perform the compression release retarding function raises the risk that loading conditions may be encountered that are different, and perhaps more severe, than were contemplated in the original design of the engine.
Since the issuance of the basic compression release patents, including the Cummins U.S. Pat. No. 3,220,392, development efforts have been directed toward improving the retarding horsepower by improving the timing of the compression release event (Custer U.S. Pat. No. 4,398,510), preventing overtravel of the slave piston (Laas U.S. Pat. No. 3,405,699), preventing overpressure of the hydraulic system (Egan U.S. Pat. No. 3,150,640), preventing overload of the injector pushtube or camshaft by release of the pressure within the high pressure circuit of the retarder mechanism (Sickler U.S. Pat. No. 4,271,796) and increasing the inlet manifold pressure during retarding (Price U.S. Pat. No. 4,296,605).
Compression release retarders are commonly actuated by an electrically operated solenoid valve provided for each cylinder or pair of cylinders in the engine. Due to this arrangement, the retarding horsepower of the engine can be varied by selectively turning on and off the retarding mechanisms associated with one or more cylinders. Thus, several levels of retarding can be provided. However, uniform or gradual variation in the retarding horsepower (at a particular engine speed) is not attained with such mechanisms.